The present invention is applicable to a number of hydrocarbon conversion processes which utilize a catalyst. For example, it is useful in the isomerization of normal butane to isobutane and the isomerization of mixed C.sub.8 aromatics, including those of high ethylbenzene content, to meta-xylene or para-xylene. The present invention may also be used in upgrading light straight run naphtha, which is a mixture rich in C.sub.5 and C.sub.6 paraffins (pentanes and hexanes), to the corresponding branched isomer, which have higher octane numbers than the feed naphtha. Another hydrocarbon conversion process in which the present invention may be used is dehydrogenation of light paraffins (C.sub.2 through C.sub.5, but primarily C.sub.3 and C.sub.4) to the corresponding olefins.
However, the most widely practiced hydrocarbon conversion process to which the present invention is applicable is catalytic reforming. Therefore, the discussion of the invention contained herein will be in reference to its application to a catalytic reforming reaction system. It is not intended that such discussion limit the scope of the invention as set forth in the claims.
Catalytic reforming is a well-established hydrocarbon conversion process employed in the petroleum refining industry for improving the octane quality of hydrocarbon feedstocks, the primary product of reforming being motor gasoline. The art of catalytic reforming is well known and does not require detailed description herein.
Briefly, in catalytic reforming, a feedstock is admixed with a recycle stream comprising hydrogen and contacted with catalyst in a reaction zone. The usual feedstock for catalytic reforming is a petroleum fraction known as naphtha and having an initial boiling point of about 180.degree. F. (355K) and an end boiling point of about 400.degree. F. (477K). The catalytic reforming process is particularly applicable to the treatment of straight run gasolines comprised of relatively large concentrations of naphthenic and substantially straight chain paraffinic hydrocarbons, which are subject to aromatization through dehydrogenation and/or cyclization reactions.
Reforming may be defined as the total effect produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics, dehydrogenation of paraffins to yield olefins, dehydrocyclization of paraffins and olefins to yield aromatics, isomerization of n-paraffins, isomerizaton of alkylcycloparaffins to yield cyclohexanes, isomerization of substituted aromatics, and hydrocracking of paraffins. Further information on reforming processes may be found in, for example, U.S. Pat. Nos. 4,119,526 (Peters et al.); 4,409,095 (Peters); and 4,440,626 (Winter et al.).
A catalytic reforming reaction is normally effected in the presence of catalyst particles comprised of one or more Group VIII nobel metals (e.g. platinum, iridium, rhodium, palladium) and a halogen combined with a porous carrier, such as a refractory inorganic oxide. Alumina is a commonly used carrier. The halogen is normally chlorine. The particles are usually spheroidal and have a diameter of from about 1/16th to about 1/8 inch (1.5-3.1 mm), though they may be as large as 1/4th inch (6.35 mm). In a particular regenerator, however, it is desirable to use catalyst particles which fall in a relatively narrow size range. A preferred catalyst particle diameter is 1/16 inch (3.1 mm). During the course of a reforming reaction, catalyst particles become deactivated as a result of mechanisms such as the deposition of coke on the particles; that is, after a period of time in use, the ability of catalyst particles to promote reforming reactions decreases to the point that the catalyst is no longer useful. The catalyst must be reconditioned, or regenerated, before it can be reused in a reforming process.
There are several basic process schemes. The catalyst in the reaction zone may be maintained in continuous use over an extended period of time, from about five months to about a year or more, depending on the quality of the catalyst and the nature of the feedstock. Following the extended period of operation the reforming reactor, or reactors, must be taken out of service while the catalyst is regenerated or replaced with fresh catalyst. Of course, this necessitates shutdown of all equipment in the reforming unit.
In another process scheme for reforming, known as the swing reactor method, catalyst is regenerated with greater frequency. A multiple fixed bed reactor system is arranged for serial flow of feedstock in such a manner that one reactor at a time can be taken off-stream while the catalyst in that reactor is regenerated or replaced with fresh catalyst. The reactor with fresh catalyst is placed on-stream when another reactor is taken off-stream for the catalyst bed to be regenerated or replaced with fresh catalyst.
In another process scheme, a moving bed reaction zone and regeneration zone are employed. The present invention is applicable to a moving bed regeneration zone. Fresh catalyst particles are fed to a reaction zone, which may be comprised of several subzones, and the particles flow through the zone by gravity. Catalyst is withdrawn from the bottom of the reaction zone and transported to a regeneration zone where a multistep process is used to recondition the catalyst to restore its full reaction promoting ability. Catalyst flows by gravity through the various regeneration steps and then is withdrawn from the regeneration zone and furnished to the reaction zone. Movement of catalyst through the zones is often referred to as continuous though, in practice, it is semi-continuous. By semi-continuous movement is meant the repeated transfer of relatively small amounts of catalyst at closely spaced points in time. For example, one batch per minute may be withdrawn from the bottom of a reaction zone and withdrawal may take one-half minute, that is, catalyst will flow for one-half-minute. If the inventory in the reaction zone is large, the catalyst bed may be considered to be continuously moving. This method of operation is preferred by many of those skilled in the art. When the moving bed method is used, there is no loss of production while catalyst is removed and replaced. Also, use of the moving bed method avoids the shutdown and start-up procedures of the swing reactor system relating to insertion and removal of a reactor in the process stream.
Catalytic reforming is the traditional octane controller in a refinery, that is, the catalytic reforming process is adjusted to vary the octane rating of the reformate product. For example, increasing temperature in a catalytic reforming zone results in a reformate of increased octane rating. Other hydrocarbons are blended with reformate in the production of motor gasoline, but the octane rating of the refinger motor gasoline pool is determined primarily by the octane rating of reformate.
Lead-based compounds may be added to gasoline in order to enhance octane. The government mandated reduction of lead content of "regular" gasoline and the total elimination of lead in the major portion of gasoline used in this country has caused refiners to look for method of increasing the octane rating of reformate. Higher octane reformate may be used to compensate for the reduction or total elimination of the lead-based additives used in motor gasoline for increasing octane rating. A method of increasing reformate octane rating is to increase the severity of the reaction. Increasing reaction temperature increases severity, as does lowering the pressure at which the reaction takes place. Reducing the hydrogen to hydrocarbon ratio also promotes greater severity. It should be noted that increasing the severity of the reaction results in a higher coke make, that is, a higher rate of deposition of coke on the catalyst in the reaction zone. A higher rate of coke deposition requires that catalyst regeneration be accomplished at a more rapid rate.
It can be seen that severity is an umbrella term which covers changes in a number of parameters. Severity is also a relative term and not susceptible of precise definition. Much depends upon the original design of a unit. For example, a refining process operating at higher than design temperature is often said to be operating at an increased severity. Another similar unit operating at the same temperature, but where that temperature is the design temperature, is often not considered to be operating at high severity. A change in catalyst or feedstock may induce one skilled in the art to refer to operation at increased severity. For example, in a catalytic reforming unit, a change from the normal feedstock to a thermally cracked naphtha feedstock would normally be considered to be a change to a higher severity operation. Many skilled in the art would consider operation of a catalytic reforming zone at a temperature above about 1020 degrees Fahrenheit (822K) to be a high severity process. In a like manner, operating at a pressure below 100 psig (791 kPaa) might be considered to be a high severity process.